For the conventional iron-base, nickel-base, cobalt-base, or chromium-base alloy, a large amount of an alloying element, such as molybdenum or tungsten, is incorporated in order to impart heat resistance. Significant segregation of these additive elements results in deteriorated hot workability and makes it difficult to produce members in a good yield. For this reason, a method has been developed which comprises the steps of: rapidly solidifying a material having the same constituents as described above by gas atomization or the like to powder the material, thereby minimizing the segregation; encapsulating the resultant alloy powder; and performing consolidation and molding by rolling, forging, HIP (high temperature hydrostatic compression), hot extrusion or the like.
The powder metallurgy alloy produced by consolidation and molding in this way, however, has a problem that, as compared with the material, having the same constituents, produced by the conventional forging-hot working process, the strength decreases with increasing the service temperature due to lower grain size or the like.
For this reason, a material produced by consolidation and molding of an alloy powder, produced by mechanically alloying an oxide powder, such as yttria, with a mother alloy by means of a ball mill or the like has been used on a commercial scale. The mechanical alloying method requires a treatment time of several tens of hours in order to offer good properties, posing problems such as increased cost and increased quality variation.
Japanese Patent Laid-Open No. 13008/1996 discloses a process for producing a fine powder of an oxide dispersion strengthened alloy, wherein reinforcing particles of an oxide are incorporated into a molten bath of a mother alloy followed by atomization. However, it is difficult to continuously and steadily introduce the oxide during the atomization, so that in fact many technical problems to be solved are left.